Modern cellular, satellite and other communications systems use multiple channels, closely spaced over an assigned communications band. In order to avoid inter-channel interference, it is essential that RF power amplifiers that are used in base station transmitters be highly linear. Non-linearities can cause signals from one channel to spread spectrally into neighboring channels, causing unacceptable interference. For this reason, cellular communications standards commonly require that inter-channel rejection be at least 60-75 dBc, i.e., that signals spreading into any given channel from other channels be at least 60-75 dB below the channel carrier. The larger the number of carriers to be amplified by the power amplifier, the more difficult it generally becomes to maintain adequate amplifier linearity.
FIG. 1 is a schematic block diagram showing the design of a base station transmitter 20, as is commonly used in cellular communications, based on a plurality of single-channel RF power amplifiers 30, 32. . . , 34. Signals are generated for transmission in a plurality of frequency channels by respective transceivers 22, 24. . . , 26. For cellular bands in the 800-900 MHz range, the typical bandwidth of a single channel can be anywhere from 10 kHz to 1.25 MHz. The signal in each channel is amplified by the respective amplifier 30, 32. . . , 34, so that frequency spreading and intermodulation between channels are avoided. The amplified signals are typically combined by a cavity combiner 36 and transmitted via an antenna 38. The use of single-channel power amplifiers 30, 32. . . , 34 enables transmitter 20 to satisfy the inter-channel rejection requirements of cellular systems. But the use of multiple power amplifiers and power combiner 36 substantially increases the cost and complexity and reduces the reliability, efficiency and operational flexibility of the transmitter. Thus, there would be a clear economic and technical advantage in using a single wideband power amplifier in place of the array of single-channel amplifiers and combining network shown in FIG. 1.
Various solutions have been proposed for improving linearity and reducing inter-channel effects in multi-carrier RF amplifiers. One such solution is the feedforward amplifier circuit, in which a portion of the signals at the input to the amplifier are fed forward and, following suitable amplitude and phase adjustment, are subtracted from the amplifier output to generate an error signal proportional to distortion components of the output. The portion of the circuit that generates the error signal is known as the signal-cancellation loop. The error signal is then amplified, phase-adjusted and subtracted from the amplifier output to give a corrected RF output with reduced distortion effects. This portion of the circuit is known as the error-cancellation loop.
Typically, the amplitude and phase adjustments in the signal- and error-cancellation loops are set by injecting a "pilot tone" and then varying the amplitude and phase until a desired output is obtained. The adjustments made in this manner, however, are inherently narrowband and give optimal amplifier performance only at a certain frequency.
U.S. Pat. No. 4,412,185, which is incorporated herein by reference, describes a feedforward amplifier using such an approach. A reference signal is injected into a main amplifying element in the feedforward amplifier so that it appears at an output terminal of the amplifier as through it were an amplifier-induced distortion. The characteristics of the amplifier are adaptively modified to eliminate the reference signal from the output signal.
A different approach that has been proposed to improve linearity of feedforward amplifiers is non-linear predistortion of the amplifier input, to compensate for the non-linear distortion introduced by the power amplifier itself. Because the amplifier characteristics change over time and in response to changing operating conditions, the predistortion is often applied adaptively. Typically, feedback from the amplifier output is used to control predistortion parameters.
FIG. 2 is a block diagram illustrating a multi-channel feedforward amplifier 40, as is known in the art, using pre-adjustment of the input signals to reduce distortion. Amplifier 40 is described in U.S. Pat. No. 5,157,345, which is incorporated herein by reference. Signals from multiple transceivers 22, 24. . . , 26 are fed to an input phase adjustment block 44 and gain adjustment block 48, which include a separate, respective phase adjuster 46 and variable gain component 50 for gain adjustment of each channel. The adjusted inputs are then summed together after adjustment by an adder 52 and are fed to a wideband power amplifier 54.
A portion of the input at each channel is split off by an input splitter 42, and these portions are summed by an adder 64. Following a suitable delay 68 (which includes phase negation of the signals), the summed input signals are subtracted at an adder 70 from a portion of the output of power amplifier 54, which is split off by a directional coupler 56, thus generating the feedforward error signal. The error signal is delayed by a delay 72, adjusted in phase and gain by phase adjuster 74 and variable gain component 76, and amplified by an amplifier 78. Following a delay 58, the output of power amplifier 54 is combined with the error signal by a directional coupler 82 to generate the amplified and corrected output signal.
Each of phase adjusters 46 and 74 and variable gain components 50 and 76 is adaptively controlled by a respective feedback network to suppress the distortion interference in the output signal. To control adjuster 74 and gain component 76 in the error correction loop, a single feedback network 86 receives sampled error signals from a directional coupler 80 and sampled output signals from a directional coupler 84 and generates control signals accordingly. For the multiple adjusters 46 and gain components 50 in blocks 44 and 48, however, a separate feedback network 88, 90. . . , 92 is required for each of the transceiver channels. As noted in the above-mentioned U.S. Pat. No. 5,157,345, there may be as many as 100 such channels transmitted by a typical cellular base station, and a comparable number of feedback networks. Each of networks 88, 90. . . , 92 receives a respective sampled input signal, split off by a directional coupler 62 in splitter block 60, together with sampled error signals from coupler 80, via a splitter 87. Each of these networks outputs control signals to the phase adjuster 46 and gain component 50 at the input of the respective channel.
Thus, feedforward amplifier 40 attempts to overcome the limitations of narrowband feedforward amplifiers by correcting each of the input signals separately, effectively providing a separate signal cancellation loop for each input channel. A similar approach is described in U.S. Pat. No. 4,560,945.
Other possible approaches to multi-channel linearization are described in U.S. Pat. No. 5,077,532, in which a microprocessor adjusts the feedforward circuits based on spectral analysis of the amplifier output signal; and in U.S. Pat. No. 5,455,537, in which a broadband pilot signal is used. An article by Parsons, et al., entitled "A Highly-Efficient Linear Amplifier for Satellite and Cellular Applications," in IEEE Globecom, Vol. 1 (December 1995), pp. 203-207, suggests combining analog predistortion with feedforward linearization. PCT patent publication WO98/12800 describes an RF power amplifier that combines feedforward correction with adaptive digital predistortion. An RF signal output by the power amplifier is down-converted and sampled, and the average power of this signal is used as an input to a look-up table, so as to vary the values of complex gain applied to the input signal to the amplifier and to a feedforward error signal.